need help with computer craft program for 1.16.5

hey, so I am getting an error with my computer craft program and it is saying stone.lua:3: bad argument #1 (string expected, got nil)

​

while true do

os.pullEvent("redstone")

if rs.getInput(back)then

break

end

end

while true do

turtle.detect()

​

turtle.dig()

​

os.sleep(1)

what I am trying to do is have the mining turtle activate the program when it detects a redstone signal then it will detect if there is a block in front of it and mine it

here is the code any help would be greatly appreciated